A shared communications link, e.g. a digital bus, typically incorporates features for controlling access of the signal sources to the communications link. Failure to provide adequate means for allocating access may cause unintentional collisions of signals from different signal sources that are simultaneously attempting to transmit over the shared communications link. Unexpected collisions may cause a loss of communications integrity and damage hardware that is connected to the communications line.
Various approaches to allocating access to a communications link are known, including access arbitration and priority assignment. As an example, U.S. Pat. No. 4,894,565 (Marquardt) entitled ASYNCHRONOUS DIGITAL ARBITER, issued Jan. 16, 1990, discloses an arbiter circuit for determining which of two asynchronous digital "chip select" signals will control communication with a shared random access memory (RAM). The arbitration opertion involves assigning priorities and generating "communications channel busy" signals to notify signal sources when the communications channel is in use.
As in the embodiment disclosed by Marquardt, known approaches to allocating access to a shared communications line may require numerous costly circuit components and extremely specific signal formats, e.g. chip select pulses. These requirements may undesirably limit the applicability of known circuits.